The invention described herein was made by a civil servant employee of the United States Government, and a non-civil servant employee working under a NASA contract, and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
1. Field of the Invention
The invention relates generally to the study, visualization and characterization of liquid drops. More specifically, it relates to a means of accurately determining dynamic contact angles and spreading rates of drops on a non-transparent surface.
2. Discussion of Relevant Art
The spreading of an evaporating liquid on a solid surface occurs in many practical processes, such as coating, painting, gluing, soldering, lubricating, mold filling, and many processes in thermal engineering. The typical processes involving heat transfer are film cooling, boiling, and liquid transportation in heat pipes. Most studies on liquid drop spreading have focused on nonvolatile liquid sessile drops for their simplicity, both in experimental measurements and theoretical analysis. The occurrence of liquid evaporation is, however, inevitable. This evaporation can induce convection in the drop, thought to be attributable to changes in surface tension caused by local variations in the temperature on the surface of the drop. The effect of the convection on the wetting and spreading of the drop is not clear.
A laser-shadowgraphic system has been used to simultaneously visualize the thermocapillary convection inside a volatile drop, and measure the spreading rate of the drop. Unfortunately, this system can only be used on sessile drops spreading on transparent substrates.
Prior techniques have suggested using the reflection of parallel beam on the surface of a sessile drop to measure the contact angle of a drop on a non-transparent substrate. This can work only when the surface of the liquid drop has enough reflectance.
The effects of evaporation on the spreading and contact angle of a liquid drop are important for a more complete understanding of these engineering processes and are of more practical interests to research and production personnel.
One object of the present invention is a non-intrusive method and apparatus to characterize the physical attributes of volatile, as well as non-volatile, liquid sessile drops.
Another object is to enable the instantaneous measurement of contact angle and spreading of a liquid drop in any direction.
Yet another object is to permit the determination of the effects of evaporation on the spreading and contact angle of a liquid drop.
Still another object is to enable the flow phenomena in transparent drops on a reflective solid surface to be visualized without the need to use microparticle tracers.
An additional object is to measure instant contact angle and spreading characteristics of a drop in any direction, even if the drop is non-transparent or is on a non-reflective substrate but without visualization of internal flow patterns.
The present invention relates to an apparatus and method of measuring the spreading characteristics of a liquid drop on a non-transparent surface. For a reflective solid surface, such as an aluminized glass plate or any substrate with a smooth surface having enough reflectance, the present invention not only determines the contact angles and spreading characteristics of a liquid drop in any direction simultaneously, but also allows visualization of the fluid flow inside the drop to identify the influences of the flow on the contact angles and spreading of the drop. As for non-reflective, solid surfaces or nontransparent liquid drops, the present invention is useful for measuring the contact angle and spreading characteristics of the drop in any direction, however, without the visualization of flow, if any, within the drop.
A source of white light and a laser beam are collimated. Generally, the collimated beams are partially reflected by the drop surface to form a drop-reflection image and partially pass through the liquid drop to form a reflection-refracted image of the laser beam without obstruction to the top view of the drop. The dynamic contact angle and spreading rate in all directions and the effects of the evaporation on the spreading and the contact angles can then be determined. Typically, the beam of white light and the laser beam are combined via a beam splitter before being collimated. The apparatus includes a second beam splitter through which the collimated beams change direction and are perpendicularly projected on the drop. The beams are partially reflected by the drop surface and partially pass through the drop without obstructing the top view of the drop. The drop-reflection image and the reflection-refracted image pass through a third beam splitter and are projected onto a screen. The apparatus includes means, such as two cameras, video recorders and monitors for recording and viewing the drop-reflection and shadowgraphic images and the magnified top view of the drop.